Carotenoid composition of terrestrial Cyanobacteria: response to natural light conditions in open rock habitats in Venezuela

Carotenoid contents of terrestrial cyanobacteria sampled from rock surfaces with different exposure to sunlight were studied in the Guyana-shield region of Venezuela. At the collecting locations of two table mountains Roraima-tepui (2700 m), Auyantepui (2000 m) and an inselberg near Puerto Ayacucho (80 m), the most frequent species were Stigonema ocellatum, Stigonema hormoides and Scytonema crassum, respectively. Usually, these species were associated with the unicellular cyanobacterium Gloeocapsa sanguinea. Their carotenoid and chlorophyll a contents were determined by high-performance liquid chromatography. In general, carotenoids consisted almost equally of myxolglycosides and of [beta]-carotene derivatives, such as [beta]-carotene, echinenone and canthaxanthin. On a chlorophyll a basis, myxolglycoside content increased in full exposure to solar irradiation. The relative proportions of the different [beta]-carotene derivatives appeared to be even more strongly influenced by irradiance. Whereas their total amount was increased at full exposure, the content of [beta]-carotene decreased and that of canthaxanthin increased. Maximal ratios of canthaxanthin/[beta]-carotene of around 5 were observed in the most sun-exposed samples. We conclude that acclimation of the carotenoid content to irradiance, especially the adjustment of the ratio canthaxanthin/[beta]-carotene, is a response found in many different terrestrial cyanobacterial species. However, when a natural population was artificially shaded for 6 days, short-term acclimation in pigment content was not detected. The results support a photoprotective function of canthaxanthin in terrestrial cyanobacteria under natural environmental conditions.